juju should warn on http url during add-cloud for lxd

$ /snap/bin/juju add-cloud
Cloud Types
  lxd
  maas
  manual
  oci
  openstack
  oracle
  vsphere

Select cloud type: lxd

Enter a name for your lxd cloud: lxd-remote

Enter the API endpoint url for the remote LXD server: http://x.x.x.x:8443

Auth Types
  certificate

Select one or more auth types separated by commas [certificate]:

Enter region [default]:

Enter the API endpoint url for the region [use cloud api url]:

Enter another region? (y/N):

Cloud "lxd-remote" successfully added
You may need to `juju add-credential lxd-remote' if your cloud needs additional credentials
Then you can bootstrap with 'juju bootstrap lxd-remote'

$ juju add-credential lxd-remote
Enter credential name: admin

Auth Types
  certificate
  interactive

Select auth type [interactive]:

Enter trust-password:

Enter The LXD client certificate, PEM-encoded. (optional):

Enter The LXD client key, PEM-encoded. (optional):

Enter server-cert (optional):

Loaded client cert/key from "/home/heather/.local/share/juju/lxd"
ERROR finalizing credential: invalid URL "http://x.x.x.x:8443": only HTTPS is supported
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